drm/i915: clean up media reset on gm45

Originally I've thought that this fixes up the reset issues on my
gm45, but that was just a red herring due to b0rked testing.

Still I much prefer writing the right values (all other fields are
reserved) instead of potentially dragging gunk around. Hence also
clear the register to 0 after a reset.

Note that Cspec is a bit confused and doesn't explicitly say that all
the other bits in this register are "reserved, mbz" like usually.
Instead they're marked as "r/o, default value = 0" which semantically
amounts to the same thing.

v2: Stop claiming this fixes anything and return 0 if successful
instead of stack garbage.

v3: Pimp the commit message to explain exactly why I think the docs
allow us to ditch the rmw cycle, spurred by a discussion with Chris.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -798,28 +798,29 @@ static int i965_reset_complete(struct drm_device *dev)
 static int i965_do_reset(struct drm_device *dev)
 {
 	int ret;
-	u8 gdrst;
 
 	/*
 	 * Set the domains we want to reset (GRDOM/bits 2 and 3) as
 	 * well as the reset bit (GR/bit 0).  Setting the GR bit
 	 * triggers the reset; when done, the hardware will clear it.
 	 */
-	pci_read_config_byte(dev->pdev, I965_GDRST, &gdrst);
 	pci_write_config_byte(dev->pdev, I965_GDRST,
-			      gdrst | GRDOM_RENDER |
-			      GRDOM_RESET_ENABLE);
+			      GRDOM_RENDER | GRDOM_RESET_ENABLE);
 	ret =  wait_for(i965_reset_complete(dev), 500);
 	if (ret)
 		return ret;
 
 	/* We can't reset render&media without also resetting display ... */
-	pci_read_config_byte(dev->pdev, I965_GDRST, &gdrst);
 	pci_write_config_byte(dev->pdev, I965_GDRST,
-			      gdrst | GRDOM_MEDIA |
-			      GRDOM_RESET_ENABLE);
+			      GRDOM_MEDIA | GRDOM_RESET_ENABLE);
 
-	return wait_for(i965_reset_complete(dev), 500);
+	ret =  wait_for(i965_reset_complete(dev), 500);
+	if (ret)
+		return ret;
+
+	pci_write_config_byte(dev->pdev, I965_GDRST, 0);
+
+	return 0;
 }
 
 static int ironlake_do_reset(struct drm_device *dev)